Electrolytic capacitors are often formed from valve action materials that are capable of being oxidized to form a dielectric layer. Typical valve action metals are niobium and tantalum. More recently, capacitors have been developed that employ an anode made from an electrically conductive oxide of niobium and a niobium pentoxide dielectric. Niobium oxide based capacitors have significant advantages over tantalum capacitors. For example, niobium oxide is more widely available and potentially less expensive to process than tantalum. Niobium oxide capacitors are also more stable against further oxidation and thus less prone to thermal runaway when over-voltaged (or otherwise over-loaded) than tantalum and niobium. Further, niobium oxide has several orders of magnitude higher minimum ignition energy compared to niobium and tantalum. Niobium oxide capacitors may also have a unique high resistance failure mechanism that limits the leakage current to a level below the capacitor's thermal runaway point.
Despite the many benefits afforded by such ceramic-based (e.g., niobium oxide) capacitors, their use in high voltage applications (e.g., rated voltage of 16, 20 or 35 volts) has often been limited due to the relatively low breakdown strength of the dielectric. Generally speaking, as the charge and voltage on a capacitor is increased, free electrons will eventually become accelerated to velocities that can liberate additional electrons during collisions with neutral atoms or molecules in a process called avalanche breakdown. Breakdown occurs quite abruptly (typically in nanoseconds), resulting in the formation of an electrically conductive path and a disruptive discharge through the material. Capacitor damage or destruction can occur in such situations. In niobium oxide capacitors, it is believed that the oxygen vacancies of Nb2O5-x act as electron donors that cause the anodic oxide to function as an n-type semiconductor. Unfortunately, it is believed that Schottky-type point defects are prevalent in the n-type semiconductor in which oxygen atoms leave their site in the lattice structure (thereby creating an oxygen vacancy) and move to the areas with lower concentration. In the case of niobium oxide capacitors, it is believed that the oxygen gradient on the interface between the niobium monoxide anode and niobium pentoxide dielectric drives the oxygen atoms to diffuse into the areas with lower concentration in the niobium monoxide, thereby creating oxygen vacancies in the dielectric. Those defects may form deep traps in the dielectric, which can store the electrical charge and serve as the source of charge carrier transport by Poole-Frenkel and tunneling mechanisms under the application of DC voltage. The application of high voltage and temperature further accelerates the oxygen diffusion and increases the number of defects in dielectric. This leads to leakage current instability at accelerated temperature and voltage load, which may limit the use of such capacitors at higher application voltages.
As such, a need currently exists for an electrolytic capacitor formed from a ceramic-based anode that is able to operate at relatively high voltages.